The present invention relates to a honeycomb structure used in, for example, a substrate for a catalyst utilizing a catalytic action, for use in an internal combustion engine, a boiler, a chemical reactor, a fuel cell reformer, etc., and a filter for trapping particulate present in an exhaust gas; as well as to an assembly thereof. More particularly, the present invention relates to a honeycomb structure that has better durability for thermal stress in use and has less pressure loss feature, as well as relates to an assembly thereof.
Honeycomb structures are in use in, for example, a carrier for a catalyst having a catalytic action, for use in an internal combustion engine, a boiler, a chemical reactor, a fuel cell reformer, etc., and a filter for trapping particulate present in an exhaust gas, particularly particulate emitted from a diesel engine.
In the honeycomb structure used for such a purpose, the rapid temperature change of exhaust gas and the local heating makes non-uniform the temperature distribution inside the honeycomb structure, and there have been problems such as crack generation in honeycomb structure and the like. When the honeycomb structure is used particularly as a filter for trapping a particulate substance in an exhaust gas emitted from a diesel engine, it is necessary to burn the fine carbon particles deposited on the filter to remove the particles and regenerate the filter and, in that case, high temperatures are inevitably generated locally in the filter; as a result, this process tends to generate large thermal stress and cracks.
Hence, there were proposed processes for producing a honeycomb structure by bonding a plurality of individual segments using an adhesive.
For example, U.S. Pat. No. 4,335,783 discloses a process for producing a honeycomb structure, which comprises bonding a large number of honeycomb parts using a discontinuous adhesive. JP-B-61-51240 proposes a thermal shock resistant rotary regenerating thermal exchanging system which comprises forming, by extrusion, matrix segments of honeycomb structure made of a ceramic material, firing them, making smooth, by processing, the outer peripheral portions of the fired segments, coating the to-be-bonded areas of the resulting segments with a ceramic adhesive having, when fired, substantially the same chemical composition as the matrix segments and showing a difference in thermal expansion coefficient, of 0.1% or less at 800° C., and firing the coated segments. SAE paper 860008 of 1986 discloses a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with a cordierite cement. JP-A-8-28246 discloses a ceramic honeycomb structure obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles.
Meanwhile, the regulation for exhaust gas has become stricter and engines have come to have higher performance. As a result, in order to achieve an improvement in combustion conditions of an engine and an increase in purification ability of a catalyst, the temperature of exhaust gas has increased year by year. In this connection, a higher thermal shock resistance has come to be required for honeycomb substrates. Therefore, even with honeycomb structures such as mentioned above, when a sharp temperature change of inflow gas takes place, and the local heat of reaction, the local heat of combustion, etc., become larger during use, it is considered to be possible that the thermal stress applied thereto is not sufficiently relaxed, cracks appear therein and, in an extreme case, there occur, for example, the disintegration of honeycomb structure and the breakage of the structure into fine pieces caused by vibration.
As means for solving the problem, there is a method of increasing a heat capacity of the honeycomb structure to reduce a temperature change, decelerating reaction and/or combustion rate, and lowering maximum temperature for relaxation of the thermal stress on the honeycomb structure. However, this method has disadvantages that reactivity, purification efficiency, and regeneration efficiency of the honeycomb structure drop and a pressure loss increases. With the use for purifying an automobile exhaust gas, there occur problems of deterioration of fuel consumption and drivability, and size enlargement of auxiliary devices. Moreover, in JP-B-54-110189, a structure has been proposed in which thickness of partition walls is regularly reduced toward a center of a cross section of a honeycomb substrate. Further in JP-A-54-150406 and JP-A-55-147154, a structure has been proposed in which cell walls of an outer peripheral portion of the honeycomb structure is thicker than that of an inner portion. Although such a honeycomb structure is strong against mechanical stress from the outside, the structure cannot be said to have sufficient durability against the thermal stress in use because of the thin inner cell walls.
The present invention has been developed in view of these conventional situations, and an object thereof is to provide a honeycomb structure which can prevent deterioration of reactivity, purification efficiency, regeneration efficiency, and the like in use and can lower pressure loss and can be better in durability against breakage by thermal stress.